The invention relates to a foil of chitosan for wound dressing.
Chitosan with the structure formula ##STR1## is a deacetylation product of chitin which can be obtained on a technical scale from the shells of crabs, crawfish and other shell-encased organisms. Chitosan is soluble in diluted solutions or organic acids, for instance in three to five percent acetic acid and can be processed from this solution to foils, plates, gels, sprays and powders, according to known precipitation processes (Manufacturing Chemist 10 (1984) pages 47 to 52).
There are various possibilities to use chitosan as a biocompatible substance in field of medical techniques. From High-Tech 7 (1988) pages 29 to 37 it is known to use chitosan foils for the wound-sealing of burns. On a wounded surface of the human body the foil behaves like a heterologous skin substitute, which not only protects the wound against undesired external mechanical action, but also creates a favorable physiological environment for healing. As a rule the foil is left as a seal on the wound until towards the end of the wound-healing phase the foil is spontaneously rejected. It has also passed through the shrinking process, respectively the reduction process of the wound and has become a kind of scab, underneath of which the injured skin was able to regenerate.
The chitosan foil has a certain air permeability. Through tests which are not part of the prior art it has been found that the oxygen-permeability of the chitosan foil is heavily dependent on the relative humidity of the environment. The oxygen permeability is drastically reduced when the relative humidity decreases. Especially when wounds having a large surface are sealed the sufficient breathing of the wound is not insured. However the gas exchange through a wound dressing is important, because a high CO.sub.2 pressure reduces the pH value and slows down the healing process, and in addition a low oxygen concentration makes possible the proliferation of anaerobic bacteria. Further the use of the known chitosan foil in the dressing of wounds with a large surface is limited due to the fact that the dry foil is relatively brittle. Large-surface wound-sealing on curved body parts, such as elbow, chin, heel, knee or shoulder portions is used rarely or not at all, because of the brittleness of the known chitosan foil.